Activated carbon is the preferred material for use in preparing electrodes for carbon electrode capacitors. This activated carbon is prepared from a number of different sources such as coconut shells, wood, sugar, cellulosics and phenolic resins. After converting these materials to carbon under steam controlled conditions, the carbons are “activated” in a second step using steam or catalyzed with KOH, NaOH and/or carbon dioxide and KOH to increase the surface area to very high surface areas such as 1000 to 2400 m2/g. These activated carbons usually contain about 2% oxygen after they have been thoroughly dried and traces of inorganic salts. This oxygen is probably present as quinones, hydroquinones, esters, phenols, carboxylic acids, furans and possibly ketones etc. with some nitrogen compounds present—all of which under higher voltage conditions greater than 3 V. will undergo electrochemical oxidation/reduction as the voltage is increased past 3.3 V. At lower voltages, these functional groups actually improve the energy storage capacity of the carbon and are desirable at voltages below 3.2 V.
The basic components of electrical capacitors include conductive electrodes connected to an electric power supply and a dielectric material separating the electrodes. Electrolytic capacitors and electrochemical double layer capacitors also have an electrolyte. In an electrolytic capacitor, the electrodes are provided by an oxide or carbon layer formed on metal foil and separated by a porous non conducting membrane such as paper, porous polymer, etc. The liquid electrolyte provides electrical contact to the opposite electrode through the separator. The inherently high resistance of electrolytic capacitors is generally mitigated by rolling a large sheet of the electrode material into a roll to give high surface area. In an electrochemical double layer capacitor, the dielectric is provided by the electrolyte. In this type of capacitor, the resistance of the electrolyte is a significant factor in the total device resistance. In capacitors that use electrolytes, the temperature has a major influence on the electrolyte in the performance of the capacitor since the conductivity of the electrolyte decreases with temperature.
Electrochemical double layer capacitors, including super capacitors, typically comprise electrodes, electrical contacts to a power supply, separators for electrodes and/or cells, an electrolyte and environmental seals. As mentioned above, a key component of electrolytic and electrochemical double layer capacitors is the electrolyte, which typically comprises a combination of a conductive salt and a solvent. Desirable electrolytes are typically liquid with low viscosity, low density, and high conductivity over a range of ambient temperature conditions. They should also be commercially inexpensive, chemically and electrochemically stable, and compatible with carbon. Aqueous electrolyte systems have been used extensively and provide voltage restricted below 1.8v. However, some electrolyte liquid systems are less effective in providing higher energy densities at lower temperatures. The current non-aqueous aprotic solvent used for ultra capacitor electrolytes is acetonitrile which is toxic, highly flammable and has a voltage limit of 2.7v. For example, ultra capacitors in Japan are not permitted to use acetonitrile for the electrolyte. A need exists for improved electrolyte systems that provide optimum capacitance for capacitors to achieve high power density, a wide temperature range, and a long lifetime without memory effects.
The key requirements for the electrolyte in both non-aqueous batteries and capacitors are high voltage stability, low temperature performance and electrochemical stability. U.S. Pat. No. 6,743,947 to Xu et al discloses an electrolyte system comprising a mixture of ethylene carbonate and dimethyl carbonate at a concentration of the salt at 0.5-2.5 M which has poor conductivity at low temperatures.
U.S. Pat. No. 5,418,682 to Warren et al, which is herein incorporated by reference discloses a method of preparing tetraalkyl ammonium tetrafluoroborate salts for use as electrolytes with dinitrile mixtures as solvents.
U.S. Pat. No. 5,965,054 to McEwen et al, which is herein incorporated by reference discloses non-aqueous electrolytes for electrical storage devices utilizing salts consisting of alkyl substituted, cyclic delocalized aromatic cations and their perfluoro derivatives with alkyl carbonate solvents.
U.S. Pat. No. 6,902,683 to Smith et al, which is herein incorporated by reference relates to electrolytes of a complex salt formed by mixing of a tetraalkyl ammonium salt of hydrogen fluoride with an imidazolium compound in a nitrile solvent which operate at temperatures between −60 and 150° C.
The article of Ue in J. electrochem. Soc. Vol 141, No. 11, November 1994 entitled “Electrochemical Properties of Organic Liquid Electrolytes Based on Quaternary Onium Salts for Electrical Double-Layer Capacitors” which is herein incorporated by reference, discloses high permittivity solvents and onium salts for double-layer capacitors. Specifically studied were quaternary onium tetrafluoroborate salts which showed greater solubility in the solvents with good stability and conductivity.